If you're trying to beat it for the first time to get the achievement/dice and are prioritizing safety, LIFE CLERIC. Best healer in the game, and Preserve Life is the best mass healing effect you can get. You won't have to use it often, but when you do, it carries.
At L12 Shadowheart was mass healing the party for 40+ health each.
Life Clerics also get a lot of the best spells already prepared (Bless, Guardian of Faith, Greater Restoration, Revivify) so you don't have to use spell preparation slots on them.
Having the right equipment matters a lot, arguably more than your build in Act III. If you're playing Honour Mode I assume you're decent at builds, many things are viable, doesn't have to be min-maxed, just capable.
But if we're talking about safety and not losing a run, yeah, Life Cleric with
Blood of Lathander. Lathander's Blessing saved my run against Ansur, most dangerous Honour Mode boss IMO). Also helps against almost all of Act II and Cazador in Act III.
For damage, honestly the MVP of my run was Karlach with a basic Berzerker Barbarian/Fighter 2 build. Once you get
on her, it's a wrap. She shredded
Even with the haste nerf, I'd still recommend using haste for attack-focused builds. Even the one extra attack makes a big difference.
If there's one dip I'd recommend, it's two levels of Fighter because Action Surge is the only thing that still gives you two extra attacks. My Tav was Blade Pact Warlock 5/Assassin Rogue 5/Fighter 2.
